3,639 research outputs found
Multimode model for an atomic Bose-Einstein condensate in a ring-shaped optical lattice
We study the population dynamics of a ring-shaped optical lattice with a high
number of particles per site and a low, below ten, number of wells. Using a
localized on-site basis defined in terms of stationary states, we were able to
construct a multiple-mode model depending on relevant hopping and on-site
energy parameters. We show that in case of two wells, our model corresponds
exactly to the latest improvement of the two-mode model. We derive a formula
for the self-trapping period, which turns out to be chiefly ruled by the
on-site interaction energy parameter. By comparing to time dependent
Gross-Pitaevskii simulations, we show that the multimode model results can be
enhanced in a remarkable way over all the regimes by only renormalizing such a
parameter. Finally, using a different approach which involves only the ground
state density, we derive an effective interaction energy parameter that shows
to be in accordance with the renormalized one.Comment: 18 pages, 12 figure
Memory effects in superfluid vortex dynamics
The dissipative dynamics of a vortex line in a superfluid is investigated
within the frame of a non-Markovian quantal Brownian motion model. Our starting
point is a recently proposed interaction Hamiltonian between the vortex and the
superfluid quasiparticle excitations, which is generalized to incorporate the
effect of scattering from fermion impurities (He atoms). Thus, a
non-Markovian equation of motion for the mean value of the vortex position
operator is derived within a weak-coupling approximation. Such an equation is
shown to yield, in the Markovian and elastic scattering limits, a He
contribution to the longitudinal friction coefficient equivalent to that
arising from the Rayfield-Reif formula. Simultaneous Markov and elastic
scattering limits are found, however, to be incompatible, since an unexpected
breakdown of the Markovian approximation is detected at low cyclotron
frequencies. Then, a non-Markovian expression for the longitudinal friction
coefficient is derived and computed as a function of temperature and He
concentration. Such calculations show that cyclotron frequencies within the
range 0.010.03 ps yield a very good agreement to the longitudinal
friction figures computed from the Iordanskii and Rayfield-Reif formulas for
pure He, up to temperatures near 1 K. A similar performance is found for
nonvanishing He concentrations, where the comparison is also shown to be
very favorable with respect to the available experimental data. Memory effects
are shown to be weak and increasing with temperature and concentration.Comment: Incidence of radiation damping analyzed in Sections I and IV C (2
references added). Derivation of the vortex equation of motion moved to an
appendix; other minor changes about style and presentation. 13 pages, no
figures. Accepted for publication in the Journal of Low Temperature Physic
Dynamics in asymmetric double-well condensates
The dynamics of Bose-Einstein condensates in asymmetric double-wells is
studied. We construct a two-mode model and analyze the properties of the
corresponding phase-space diagram, showing in particular that the minimum of
the phase-space portrait becomes shifted from the origin as a consequence of
the nonvanishing overlap between the ground and excited states from which the
localized states are derived. We further incorporate effective interaction
corrections in the set of two-mode model parameters. Such a formalism is
applied to a recent experimentally explored system, which is confined by a
toroidal trap with radial barriers forming an arbitrary angle between them. We
confront the model results with Gross-Pitaevskii simulations for various angle
values finding a very good agreement. We also analyze the accuracy of a
previously employed simple model for moving barriers, exploring a possible
improvement that could cover a wider range of trap asymmetries.Comment: 15 pages, 11 figure
Laboratory evaluation of a pilot cell battery protection system for photovoltaic applications
An energy storage method for the 3.5 kW battery power system was investigated. The Pilot Cell Battery Protection System was tested for use in photovoltaic power systems and results show that this is a viable method of storage battery control. The method of limiting battery depth of discharge has the following advantages: (1) temperature sensitivity; (2) rate sensitivity; and (3) state of charge indication. The pilot cell concept is of interest in remote stand alone photovoltaic power systems. The battery can be protected from damaging overdischarge by using the proper ratio of pilot cell capacities to main battery capacity
Dark soliton collisions in a toroidal Bose-Einstein condensate
We study the dynamics of two gray solitons in a Bose-Einstein condensate
confined by a toroidal trap with a tight confinement in the radial direction.
Gross-Pitaevskii simulations show that solitons can be long living objects
passing through many collisional processes. We have observed quite different
behaviors depending on the soliton velocity. Very slow solitons, obtained by
perturbing the stationary solitonic profile, move with a constant angular
velocity until they collide elastically and move in the opposite direction
without showing any sign of lowering their energy. In this case the density
notches are always well separated and the fronts are sharp and straight. Faster
solitons present vortices around the notches, which play a central role during
the collisions. We have found that in these processes the solitons lose energy,
as the outgoing velocity turns out to be larger than the incoming one. To study
the dynamics, we model the gray soliton state with a free parameter that is
related to the soliton velocity. We further analyze the energy, soliton
velocity and turning points in terms of such a free parameter, finding that the
main features are in accordance with the infinite one-dimensional system.Comment: 15 pages, 11 figures. Accepted in PR
A search for hydrogenated fullerenes in fullerene-containing planetary nebulae
Detections of C60 and C70 fullerenes in planetary nebulae (PNe) of the
Magellanic Clouds and of our own Galaxy have raised the idea that other forms
of carbon such as hydrogenated fullerenes (fulleranes like C60H36 and C60H18),
buckyonions, and carbon nanotubes, may be widespread in the Universe. Here we
present VLT/ISAAC spectra (R ~600) in the 2.9-4.1 microns spectral region for
the Galactic PNe Tc 1 and M 1-20, which have been used to search for
fullerene-based molecules in their fullerene-rich circumstellar environments.
We report the non-detection of the most intense infrared bands of several
fulleranes around ~3.4-3.6 microns in both PNe. We conclude that if fulleranes
are present in the fullerene-containing circumstellar environments of these
PNe, then they seem to be by far less abundant than C60 and C70. Our
non-detections together with the (tentative) fulleranes detection in the
proto-PN IRAS 01005+7910 suggest that fulleranes may be formed in the short
transition phase between AGB stars and PNe but they are quickly destroyed by
the UV radiation field from the central star.Comment: Accepted for publication in Astronomy & Astrophysics (7 pages, 3
figures, and 3 Tables
Friction force on a vortex due to the scattering of superfluid excitations in helium II
The longitudinal friction acting on a vortex line in superfluid He is
investigated within a simple model based on the analogy between such vortex
dynamics and that of the quantal Brownian motion of a charged point particle in
a uniform magnetic field. The scattering of superfluid quasiparticle
excitations by the vortex stems from a translationally invariant interaction
potential which, expanded to first order in the vortex velocity operator, gives
rise to vortex transitions between nearest Landau levels. The corresponding
friction coefficient is shown to be, in the limit of elastic scattering
(vanishing cyclotron frequency), equivalent to that arising from the Iordanskii
formula. Proposing a simple functional form for the scattering amplitude, with
only one adjustable parameter whose value is set in order to get agreement to
the Iordanskii result for phonons, an excellent agreement is also found with
the values derived from experimental data up to temperatures about 1.5 K.
Finite values of the cyclotron frequency arising from recent theories are shown
to yield similar results. The incidence of vortex-induced quasiparticle
transitions on the friction process is estimated to be, in the roton dominated
regime, about 50 % of the value of the friction coefficient, 8 % of which
corresponds to roton-phonon transitions and 42 % to roton
ones.Comment: 15 pages, 4 figures; typos corrected, to be published in PR
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